This invention relates to mixtures of resorcinol (also known as resorcin or 1,3-benzenediol) and silica, which mixtures are used as adhesion promoters in rubber mixtures and synthetic plastic mixtures. The invention also relates to the silicas which can be used in these mixtures.
The use of adhesion reinforcers to improve the adhesion between rubber and textile strength carriers, such as polyamides, aramides, celluloses, and polyesters or bare, galvanized, or brass-coated steel cord is known.
There are two possibilities available for the direct addition of the adhesion reinforcers to the raw rubber mixture (i.e., dry bonding). In the case of steel cord (e.g., steel belts in tires, high-pressure hoses, or conveyor belts with steel inserts), adhesion reinforcers in the form of the divalent salts of cobalt (e.g., cobalt naphthenate) or resorcinol/formaldehyde/silica systems are used. In the case of textile cord, only the resorcinol/formaldehyde/silica system is used as adhesion reinforcement.
Both of these systems exhibit various disadvantages. The cobalt based system has poor ageing behavior. The resorcinol/formaldehyde/silica system for rubber has poisoning problems. Furthermore, the resorcinol based system has disadvantages in that the resorcinol tends to sublime, is toxic, and has poor dispersability.
In the early stages of the development of the resorcinol/formaldehyde/silica system, there was the problem of the poor dispersing behavior of resorcinol. It was known that this could be improved only by utilizing an incorporation temperature of approximately 120.degree. C. (i.e., above the melting point of resorcinol). However, these high temperatures are not achieved by all mixtures in practice. One possibility for solving this problem included the fine grinding of resorcinol with silica, as described in German Patent No. 13 01 478 (Degussa adhesive system Cofill.RTM. 11), or in the grinding of resorcinol with stearic acid. This German patent is entirely incorporated herein by reference. In addition, an attempt was made to improve the dispersion of resorcinol by means of a suitable mixing technique and mixing units, and in this manner, to use resorcinol in pure form for economic reasons.
However, these measures were not able to eliminate the problems stemming from the sublimation tendency of resorcinol, at the temperatures which are necessary for the production of articles of rubber and plastic. Since it can not be precisely determined in practice how much of the resorcinol added to the mixture will evaporate due to sublimation, sharp variations and inhomogeneities in the adhesion values can not be avoided.
Moreover, there are problems and health risks posed for employees due to the presence of sublimating resorcinol. These problems and risks must be considered. Resorcinol, especially resorcinol vapors, are regarded as injurious to the health. They have an irritating effect on the respiratory system and result in irritations of the eyes and skin (based on the disclosure from the British Rubber Manufacturers Association Ltd., Toxicity and Safe Handling of Rubber Chemicals, Third Edition, 1990). Therefore, the rubber industry increasingly requires measures which counteract the sublimation tendency of resorcinol from the manufacturers of resorcinol and of adhesive systems based on resorcinol. A development intended to aid this problem is the use of so-called resorcinol precondensates. A resorcinol resin is formed thereby by means of a purposeful reaction of resorcinol with a suitable formaldehyde donor outside of the actual mixing production.
This purposeful increase in the molecular weight of resorcinol (di-resorcinol, tri-resorcinol and oligomeric resorcinol developed from the monomeric parent substance) sharply reduces the sublimation behavior of resorcinol. In the actual application of this higher-molecular weight resorcinol in rubber, the actual resorcinol-formaldehyde resin network necessary for the cord and rubber adhesion is then built up together with a further formaldehyde donor. However, the precondensates produced in accordance with the above-described methods exhibit the disadvantage of high cost, as well as a few serious technical disadvantages. Due to the partial occupation of the reactive centers of the resorcinol for the resinification (i.e., gumming) during the precondensation, the reactivity for the actual resorcinol formaldehyde resin formation in the rubber mixture decreases considerably, relative to the reactivity of free resorcinol. This distinctly affects the vulcanization speed, results in a modulus which is distinctly lower relative to resorcinol, and a lower hardness value, as well as a lower adhesion value, especially on bare steel cord. These technical deficiencies oppose the use of precondensates in the rubber industry.